Melt Spinning of Flexible and Conductive Immiscible Thermoplastic/Elastomer Monofilament for Water Detection

In many textile fields, such as industrial structures or clothes, one way to detect a specific liquid leak is the electrical conductivity variation of a yarn. This yarn can be developed using melt spun of Conductive Polymer Composites (CPCs), which blend insulating polymer and electrically conductive fillers. This study examines the influence of the proportions of an immiscible thermoplastic/elastomer blend for its implementation and its water detection. The thermoplastic polymer used for the detection property is the polyamide 6.6 (PA6.6) filled with enough carbon nanotubes (CNT) to exceed the percolation threshold. However, the addition of fillers decreases the polymer fluidity, resulting in the difficulty to implement the CPC. Using an immiscible polymers blend with an elastomer, which is a propylene-based elastomer (PBE) permits to increase this fluidity and to create a flexible conductive monofilament. After characterizations (morphology, rheological and mechanical) of this blend (PA6.6CNT/PBE) in different proportions, two principles of water detection are established and carried out with the monofilaments: the principle of absorption and the short circuit. It is found that the morphology of the immiscible polymer blend had a significant role in the water detection.


Introduction
In the last decade, the development of new detectors in the smart textile fields has been increasing. They are used to control physical parameters like for instance temperature variation [1], stress and deformation [2] for athletics or health fields or presence of liquids and gases [3][4][5][6][7] in industry and environmental protection [8]. This technology of smart textiles is based on the electrical conductivity variation of the material in interaction with its environment to detect and transmit the data. In the field of monitoring, more and more construction industries use composites to reinforce their concrete structures as for instance retention tanks. Concrete is a fragile material, which can cause fluid leakages, and which is sometimes dangerous for the environment (pollutant). Smart textiles can be incorporated inside the composite resin or concrete to monitor the stress and deformation of the structure and detect a fluid leakage in case of damage. Consequently, more and more researches develop intelligent composite membranes [9][10][11][12]. The intelligent composite membranes are composed of two linked parts: the matrix, which is the resin, or the concrete reinforced by textile structure including smart filaments, which detect problems.
To detect fluids with smart textiles, different technologies that are based on the electrical conductivity variation of the textile in contact with the fluid can be employed. The most commonly studied in the literature are the intrinsically conducting polymers (ICPs) [13][14][15] or the conductive polymer composites (CPCs) [16][17][18]. They have the particularity of modifying their electrical conductivity according to the affinity of the polymer with the fluid [18][19][20]. They can be presented in the form of films [21], monofilaments [22], multifilaments [6,20], or coating [23]. Regarding the ICPs, the detection of fluid results in the

Conductive Yarns
The monofilaments are composed of a blend of a thermoplastic polymer with fillers and a propylene-based elastomer (PBE).
The first blend is the thermoplastic polymer with 3 wt.% of fillers. The thermoplastic polymer is the polyamide 6.6 (PA6.6) TORZEN U4803 NC01 produced by Invista (Wichita, KS, USA), which has a melting point of 263 • C. The fillers are the multiwalled carbon nanotubes NC 7000 (MWCNT) supplied by Nanocyl (Sambreville, Belgium). These MWCNTs have an average length of approximately 1.5 µm, a diameter of 9.5 nm, and a specific area of 250-300 m 2 /g. The polyamide 6.6 has a moisture regain of about 4% [46]. Therefore, thanks to the PA6.6's affinity to water, this blend is used for the detection by the electrical conductivity variation.
The second blend is the addition of the PBE to the PA6.6 3CNT to increase the fluidity of the blend and, thus, to facilitate the compounds preparation [47]. The PBE is also used to add a flexible property to the detection yarn to not break during the resin cracking. The employed elastomer is the VISTAMAXX 3000, which is supplied by ExxonMobil Chemical (Houston, TX, USA).

Compounds Preparations
Two successive extrusions are realized in order to obtain detector yarns. First, the incorporation and the dispersion of 3 wt.% of MWCNT in the PA6.6 (PA6.6 3CNT ) are processed by a co-rotating intermeshing twin-screw extruder from Thermo-Haake PTW 16/25p (barrel length = 25:1 L/D). The second step allows to add different percentages of PBE (from 0 to 50 wt.%) in the blend ( Table 1). The second extrusion use the Process 11 Parallel Twin-Screw Extruder from Thermofischer (Waltham, MA, USA) with a barrel length of 40:1 L/D. The processing conditions was based on the study of Javadi Toghchi et al. [43], which have already worked on the extrusion of the PA6.6 3CNT . The rotating speed of these extruders is 100 RPM, and the temperatures profiles are reported in the Table 2. Before each extrusion, the polymer pellets are dried at 80 • C for 16 h.  T1  T2  T3  T4  T5  T6  T7  T8   PA6.6 3CNT  260  270  275  275  280  ---PA6.6 3CNT /PBE 215  275  285  285  278  275  270  270 The monofilaments have a diameter of approximately 1.5 mm ± 0.07 mm. For the Transmission Electron Microscopy (TEM) images, the samples are prepared by the ultramicrotomy method to have clean and flat surfaces. The different polymer phases are detectable thanks to the CNT present in the PA6.6.

Rheological Properties Characterization
The Melt Flow Index (MFI) determines the flow ability of a polymer and more precisely the flowing polymer weight in 10 min at a certain temperature. In this study, the test is executed on the melt flow tester from Thermo-Haake with a temperature of 270 • C and a pressure of 2.16 kg according to the standard ISO-11333. Before the test, the polymer pellets are dried at 80 • C for 16 h.

Mechanical Property Characterization
The elongation at break of monofilaments is measured by an MTS Criterion tensile bench from MTS (Minnesota, USA). The tests are realized with an initial length of 150 mm, initial speed of 500 mm/min and a pre-loaded of 5 N. They are executed under a controlled and conditioned atmosphere of 65% of relative humidity and a temperature of 20 • C. Five measurements for each blend are necessary to accept the results with a standard deviation of about 20%.

Water Detection Methods
To validate the results and the repeatability of the protocols, all the water detection tests are realized ten times for each monofilament and conditioned at a room temperature of 20 • C and a relative humidity of 35%. Since the detection's mechanisms depends directly on the ionic conductivity of the demineralized water, this parameter is controlled before each test with a conductimeter (Tacussel electronic type cdrv 62) and a conductivity standard solution of 1413 µS/cm at a temperature of 25 • C (KCL, Fischer Scientific, Waltham, MA, USA). The surface tension of the water is also overseen and measured by the tensiometer "3S Scales" from GBX Instruments. The demineralized water had an ionic conductivity of about 4.7 ± 0.9 µS/cm and a surface tension of about 71.8 ± 3.2 mN/m. For all the electrical conductivity tests, the monofilaments are connected to an a Keithley 2461 SourceMeter (Beaverton, OR, USA). This device measures the current intensity while applying a voltage which ranges from −0.5 V to 15 V with an increment of 0.1 V. Using the data of the current as a function of the voltage sent, the electrical conductivity for a length of 10 cm is determined by the Equation (1). The inverse of the resistance is the directing coefficient of the linear trendline of the current-voltage functions between 0 and 12 V.
where σ is the electrical conductivity of the system, L is the monofilament's length (L = 0.1 m), S is the monofilament's area (m 2 ) and R is the resistance measured (Ω).
The short circuit is based on a conductive path creation between two parallel monofilaments through the drop of water (Figure 1a). The electrical signal is detected when the water makes the link between the two electrically conductive filaments. To compare the different proportions of the PBE in the blend, the conductance is calculated (Equation (2)) from the measured resistance. A square of 3 × 3 cm of absorbent paper is added on the parallel monofilaments to deposit the drop of water to make the test repeatable. With absorbent paper, the spreading of the drop is controlled by eliminating the shape of the drop and the problems of absorption that could vary the conductance of the circuit: where G is the conductance of the circuit (S) and R is the resistance of the circuit (Ω).
Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 13 different proportions of the PBE in the blend, the conductance is calculated (Equation (2)) from the measured resistance. A square of 3 × 3 cm of absorbent paper is added on the parallel monofilaments to deposit the drop of water to make the test repeatable. With absorbent paper, the spreading of the drop is controlled by eliminating the shape of the drop and the problems of absorption that could vary the conductance of the circuit: where G is the conductance of the circuit (S) and R is the resistance of the circuit (Ω). The principle of detection by absorption ( Figure 1b) is based on the modification of the resistance of the yarn when it is soaked in water. The electrical conductivities of the dry and wetted monofilament are calculated (Equation (1)). To observe the influence of the PBE percentage on the detection, the detector sensitivity (Sw) calculated corresponds to the change in the electrical conductivity between the dry and the wetted monofilament (Equation (3)): where Sw is the detector sensitivity (%), σd represents the dry electrical conductivity (S/m), and σw is the wetted electrical conductivity (S/m). To find the optimal formulation for the CPC detector filament, a figure of merit is defined. It considers the mechanical and detection properties (Equation (4)): where F is the figure of merit (%), e is the elongation at break (%) for the mechanical property, and Sw the filament sensitivity to water (%).

Morphology of the Blends
The visualization of the SEM and TEM images are important to validate the different assumptions of the blends' behaviors. By hypothesis, the continuity of the PA6.63CNT phase is influenced by the blend proportions and the localization of the CNT in the PA6.6 is favored by the separation of the compounds process into two extrusions.
The observation of TEM image of the PA6.63CNT permits to conclude on the good dispersion of the CNT (black color on the Figure 2a) in the PA6.6 polymer (lighter background on the Figure 2a).
Regarding PA6.63CNT90/PBE10 blend, the SEM image ( Figure 2b) reveals a nodular morphology of the PBE in the PA6.63CNT. Moreover, no migration or aggregate of the CNT in the PBE or at the interface between the two polymers are detected with the TEM image ( Figure 2c). This kind of morphology and this CNT localization can be confirmed in other studies [46,48,49]. The principle of detection by absorption ( Figure 1b) is based on the modification of the resistance of the yarn when it is soaked in water. The electrical conductivities of the dry and wetted monofilament are calculated (Equation (1)). To observe the influence of the PBE percentage on the detection, the detector sensitivity (Sw) calculated corresponds to the change in the electrical conductivity between the dry and the wetted monofilament (Equation (3)): where Sw is the detector sensitivity (%), σd represents the dry electrical conductivity (S/m), and σw is the wetted electrical conductivity (S/m). To find the optimal formulation for the CPC detector filament, a figure of merit is defined. It considers the mechanical and detection properties (Equation (4)): where F is the figure of merit (%), e is the elongation at break (%) for the mechanical property, and Sw the filament sensitivity to water (%). Regarding the PA6.63CNT60/PBE40 blend, the SEM image ( Figure 2d) indicates a fibrillar morphology of the PA6.63CNT in the PBE. Therefore, the addition of 40 wt.% of PBE in the blend permits to achieve a phase inversion (Figure 3). The majority phase of PBE becomes the CPC matrix, which modify the monofilaments properties. Moreover, thanks to the TEM image (Figure 2e), the CNT (black) have not migrated into the PBE (lighter color) or aggregated at the interface as the PA6.63CNT90/PBE10 blend.   Regarding PA6.6 3CNT 90/PBE10 blend, the SEM image ( Figure 2b) reveals a nodular morphology of the PBE in the PA6.6 3CNT . Moreover, no migration or aggregate of the CNT in the PBE or at the interface between the two polymers are detected with the TEM image ( Figure 2c). This kind of morphology and this CNT localization can be confirmed in other studies [46,48,49].

Rheological Properties
Regarding the PA6.6 3CNT 60/PBE40 blend, the SEM image ( Figure 2d) indicates a fibrillar morphology of the PA6.6 3CNT in the PBE. Therefore, the addition of 40 wt.% of PBE in the blend permits to achieve a phase inversion (Figure 3). The majority phase of PBE becomes the CPC matrix, which modify the monofilaments properties. Moreover, thanks to the TEM image (Figure 2e), the CNT (black) have not migrated into the PBE (lighter color) or aggregated at the interface as the PA6.6 3CNT 90/PBE10 blend.   The influence of the proportion of CNT and PBE on the rheological properties is investigated through the Melt Flow Index (MFI) analysis ( Figure 4). The CNT decreases the blend fluidity whereas the PBE permits to increase it.
The addition of PBE does not improve the fluidity of the blend without fillers: 73 g/10 min for the PA6.6 and 66 g/10 min for PA6.6 blended with 50 wt.% of PBE. However, it permits to overcome the decrease of the fluidity due to the CNT: from 9.5 g/10 min for the PA6.63CNT to 26 g/10 min for the PA6.63CNT50/PBE50, so the fluidity of the filled blend increases with the proportion of elastomer. Therefore, it is found that the CNT increases the viscosity of the blend contrary to the PBE, which increases the fluidity of the polymers blend. The addition of CNT creates more links between the fillers and, thus, reduces the mobility of the macromolecular chains of the PA6.6 [43,48]. The PBE is, thus, adding to overcome to this high viscosity and to improve the blend implementation by melt spun [27].

Mechanical Property
The elongation at break is also studied to observe the influence of the fillers and the elastomer on the blend ( Figure 5). The addition of CNT in the blend decreases weakly the mechanical property [49] contrary to the PBE addition. The PBE permits to increase the elongation at break of the filled blends until the phase inversion.
Regarding the unfilled blends, the addition of 10 wt.% of PBE permits to increase of 23% the monofilament elongation. Beyond 10 wt.%, the weak interfacial cohesion between PA6.6 and PBE, which increases with the PBE proportion, leads to a decrease in elongation.
Regarding the filled blend, the elongation at break of the monofilaments with less than 30 wt.% of PBE slightly varied from 16 to 20%. While, after 30 wt.% of PBE, the mechanical property abruptly decreases: to 4%. This mechanical property variation can be correlated with the morphology variation. Before 30 wt.%, the nodules of PBE permits a larger elongation of the monofilament before the break. The interfacial area between the two polymers has a low cohesion. Therefore, with a high percentage of PBE, the interface between the PA6.63CNT fibrils and the PBE increases and causes the premature break of the monofilament. This result is confirmed in the study of Qiu et al. [50] on a polyamide 6/polyolefin elastomer blend. The tensile strength property decreases with the addition of elastomer in the blend. They have explained that this result was expected due to the morphology of the blend and the lower tensile strength of elastomer compared with polyamide 6. Theses hypotheses are also verified with other studies on the influence of the elastomer on polycarbonate/CNT blend [45,51]. The addition of PBE does not improve the fluidity of the blend without fillers: 73 g/10 min for the PA6.6 and 66 g/10 min for PA6.6 blended with 50 wt.% of PBE. However, it permits to overcome the decrease of the fluidity due to the CNT: from 9.5 g/10 min for the PA6.6 3CNT to 26 g/10 min for the PA6.6 3CNT 50/PBE50, so the fluidity of the filled blend increases with the proportion of elastomer. Therefore, it is found that the CNT increases the viscosity of the blend contrary to the PBE, which increases the fluidity of the polymers blend. The addition of CNT creates more links between the fillers and, thus, reduces the mobility of the macromolecular chains of the PA6.6 [43,48]. The PBE is, thus, adding to overcome to this high viscosity and to improve the blend implementation by melt spun [27].

Mechanical Property
The elongation at break is also studied to observe the influence of the fillers and the elastomer on the blend ( Figure 5). The addition of CNT in the blend decreases weakly the mechanical property [49] contrary to the PBE addition. The PBE permits to increase the elongation at break of the filled blends until the phase inversion.
Regarding the unfilled blends, the addition of 10 wt.% of PBE permits to increase of 23% the monofilament elongation. Beyond 10 wt.%, the weak interfacial cohesion between PA6.6 and PBE, which increases with the PBE proportion, leads to a decrease in elongation.
Regarding the filled blend, the elongation at break of the monofilaments with less than 30 wt.% of PBE slightly varied from 16 to 20%. While, after 30 wt.% of PBE, the mechanical property abruptly decreases: to 4%. This mechanical property variation can be correlated with the morphology variation. Before 30 wt.%, the nodules of PBE permits a larger elongation of the monofilament before the break. The interfacial area between the two polymers has a low cohesion. Therefore, with a high percentage of PBE, the interface between the PA6.6 3CNT fibrils and the PBE increases and causes the premature break of the monofilament. This result is confirmed in the study of Qiu et al. [50] on a polyamide 6/polyolefin elastomer blend. The tensile strength property decreases with the addition of elastomer in the blend. They have explained that this result was expected due to the morphology of the blend and the lower tensile strength of elastomer compared with polyamide 6. Theses hypotheses are also verified with other studies on the influence of the elastomer on polycarbonate/CNT blend [45,51].

Electrical Properties
The initial electrical conductivity depends on the morphology of the blend: the localization and the dispersion of the CNT and the morphology of the polymer phases.
The electrical conductivity decreases with the adding of PBE in the blend ( Figure 6): from 1.2 × 10 −2 S/m to about 2.6 × 10 −3 S/m. With a nodular morphology, the dry conductivities are approximately the same: from, respectively, 2.6 × 10 −3 S/m to 1.5 × 10 −3 S/m for 10 wt.% and 30 wt.% of PBE. However, the dry conductivity decreases suddenly to 2 × 10 −7 S/m with the percentage of PBE above 30 wt.%. The phase inversion between 30 and 40 wt.% causes the increase in the interparticular distance and, thus, the decrease of the conductivity.

Principle of Short Circuit
The short circuit's signal depends on several parameters: the distance between the two parallel monofilaments and the water properties, which are both fixed, as well as on the monofilaments' properties. In this study, the conductance of the short circuit depends on the dry conductivity of the monofilament and, thus, the proportion of PBE in the blend

Electrical Properties
The initial electrical conductivity depends on the morphology of the blend: the localization and the dispersion of the CNT and the morphology of the polymer phases.
The electrical conductivity decreases with the adding of PBE in the blend ( Figure 6): from 1.2 × 10 −2 S/m to about 2.6 × 10 −3 S/m. With a nodular morphology, the dry conductivities are approximately the same: from, respectively, 2.6 × 10 −3 S/m to 1.5 × 10 −3 S/m for 10 wt.% and 30 wt.% of PBE. However, the dry conductivity decreases suddenly to 2 × 10 −7 S/m with the percentage of PBE above 30 wt.%. The phase inversion between 30 and 40 wt.% causes the increase in the interparticular distance and, thus, the decrease of the conductivity.

Electrical Properties
The initial electrical conductivity depends on the morphology of the blend: the localization and the dispersion of the CNT and the morphology of the polymer phases.
The electrical conductivity decreases with the adding of PBE in the blend ( Figure 6): from 1.2 × 10 −2 S/m to about 2.6 × 10 −3 S/m. With a nodular morphology, the dry conductivities are approximately the same: from, respectively, 2.6 × 10 −3 S/m to 1.5 × 10 −3 S/m for 10 wt.% and 30 wt.% of PBE. However, the dry conductivity decreases suddenly to 2 × 10 −7 S/m with the percentage of PBE above 30 wt.%. The phase inversion between 30 and 40 wt.% causes the increase in the interparticular distance and, thus, the decrease of the conductivity.

Principle of Short Circuit
The short circuit's signal depends on several parameters: the distance between the two parallel monofilaments and the water properties, which are both fixed, as well as on the monofilaments' properties. In this study, the conductance of the short circuit depends on the dry conductivity of the monofilament and, thus, the proportion of PBE in the blend

Principle of Short Circuit
The short circuit's signal depends on several parameters: the distance between the two parallel monofilaments and the water properties, which are both fixed, as well as on the monofilaments' properties. In this study, the conductance of the short circuit depends on the dry conductivity of the monofilament and, thus, the proportion of PBE in the blend (Figure 7). The detection signal is better when the dry conductivity of the monofilament is high and, therefore, when the proportion of PBE is small in the blend. The signal is from about 1.4 × 10 −7 S without PBE to 8.3 × 10 −9 S with 30 wt.% of PBE in the blend, and it decreases to 1.7 × 10 −12 S with 50 wt.% of PBE in the blend. Nanomaterials 2022, 12, x FOR PEER REVIEW 9 of 13 ( Figure 7). The detection signal is better when the dry conductivity of the monofilament is high and, therefore, when the proportion of PBE is small in the blend. The signal is from about 1.4 × 10 −7 S without PBE to 8.3 × 10 −9 S with 30 wt.% of PBE in the blend, and it decreases to 1.7 × 10 −12 S with 50 wt.% of PBE in the blend.

Principle of absorption
The water detector sensitivity (Sw) of the principle of absorption is based on the variation of the yarn's conductivity. It is the change between the electrical conductivity of the dry monofilament (dry conductivity) and of the wetted monofilament (wetted conductivity). The sensitivity to water depends on the blends' formulation. Therefore, it has the same trend as the initial electrical conductivity (Figure 8). The blends with a high dry conductivity have a positive sensitivity: from 43 ± 13 to 28 ± 20% with the percentage from 0 wt.% to 30 wt.% of PBE. However, the blends with a low one have a negative sensitivity: respectively, −26 ± 22 and −51 ± 30 % for 40 and 50 wt.% of PBE. By comparing the Sw of the different blends, it is possible to make hypotheses regarding the percentage of PBE on the Sw (Figure 9). Regarding the positive Sw, the absorbed water increases the monofilament electrical conductivity by increasing the number of conductive paths between the CNT (Figure 9a). The negative Sw is due to the blend

Principle of Absorption
The water detector sensitivity (Sw) of the principle of absorption is based on the variation of the yarn's conductivity. It is the change between the electrical conductivity of the dry monofilament (dry conductivity) and of the wetted monofilament (wetted conductivity). The sensitivity to water depends on the blends' formulation. Therefore, it has the same trend as the initial electrical conductivity (Figure 8). The blends with a high dry conductivity have a positive sensitivity: from 43 ± 13 to 28 ± 20% with the percentage from 0 wt.% to 30 wt.% of PBE. However, the blends with a low one have a negative sensitivity: respectively, −26 ± 22 and −51 ± 30 % for 40 and 50 wt.% of PBE. Nanomaterials 2022, 12, x FOR PEER REVIEW 9 of 13 ( Figure 7). The detection signal is better when the dry conductivity of the monofilament is high and, therefore, when the proportion of PBE is small in the blend. The signal is from about 1.4 × 10 −7 S without PBE to 8.3 × 10 −9 S with 30 wt.% of PBE in the blend, and it decreases to 1.7 × 10 −12 S with 50 wt.% of PBE in the blend.

Principle of absorption
The water detector sensitivity (Sw) of the principle of absorption is based on the variation of the yarn's conductivity. It is the change between the electrical conductivity of the dry monofilament (dry conductivity) and of the wetted monofilament (wetted conductivity). The sensitivity to water depends on the blends' formulation. Therefore, it has the same trend as the initial electrical conductivity (Figure 8). The blends with a high dry conductivity have a positive sensitivity: from 43 ± 13 to 28 ± 20% with the percentage from 0 wt.% to 30 wt.% of PBE. However, the blends with a low one have a negative sensitivity: respectively, −26 ± 22 and −51 ± 30 % for 40 and 50 wt.% of PBE. By comparing the Sw of the different blends, it is possible to make hypotheses regarding the percentage of PBE on the Sw (Figure 9). Regarding the positive Sw, the absorbed water increases the monofilament electrical conductivity by increasing the number of conductive paths between the CNT (Figure 9a). The negative Sw is due to the blend By comparing the Sw of the different blends, it is possible to make hypotheses regarding the percentage of PBE on the Sw (Figure 9). Regarding the positive Sw, the absorbed water increases the monofilament electrical conductivity by increasing the number of conductive paths between the CNT (Figure 9a). The negative Sw is due to the blend morphology inversion and with the presence of water. By hypothesis, the water permits the swelling of the PA6.6 3CNT fibrils, resulting in the increase of the distance between the CNT, which reduces the conductivity of the monofilament (Figure 9b). Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 13 morphology inversion and with the presence of water. By hypothesis, the water permits the swelling of the PA6.63CNT fibrils, resulting in the increase of the distance between the CNT, which reduces the conductivity of the monofilament (Figure 9b). To develop the filament detector, a trade-off between good rheological and mechanical properties and a good sensitivity is needed. The figure of merit (F), which quantifies the mechanical and detection property, decreases slightly before dropping sharply for the blends with more than 30 wt.% PBE ( Figure 10). As all the properties, this fall corresponds to the phase inversion of the morphology between nodular and fibrillar. The figure of merit highlights the decrease of the elongation at break and the sensitivity to water with the addition of PBE in the blend. Moreover, to develop the detector filament using the melt spinning, the melt flow index (MFI) has to be around 20 to 25 g/10 min. The two formulations that correspond to these criteria are PA6.63CNT80/PBE20 and the PA6.63CNT70/PBE30. Therefore, the optimum CPC filament to optimize is revealed thanks to the figure of merit which is the PA6.63CNT70/PBE30.

Conclusions
This study is focused on the development and the characterization of an immiscible thermoplastic/elastomer blend for the water detection. The monofilament is created by To develop the filament detector, a trade-off between good rheological and mechanical properties and a good sensitivity is needed. The figure of merit (F), which quantifies the mechanical and detection property, decreases slightly before dropping sharply for the blends with more than 30 wt.% PBE ( Figure 10). As all the properties, this fall corresponds to the phase inversion of the morphology between nodular and fibrillar. The figure of merit highlights the decrease of the elongation at break and the sensitivity to water with the addition of PBE in the blend. Moreover, to develop the detector filament using the melt spinning, the melt flow index (MFI) has to be around 20 to 25 g/10 min. The two formulations that correspond to these criteria are PA6.6 3CNT 80/PBE20 and the PA6.6 3CNT 70/PBE30. Therefore, the optimum CPC filament to optimize is revealed thanks to the figure of merit which is the PA6.6 3CNT 70/PBE30. Nanomaterials 2022, 12, x FOR PEER REVIEW 10 of 13 morphology inversion and with the presence of water. By hypothesis, the water permits the swelling of the PA6.63CNT fibrils, resulting in the increase of the distance between the CNT, which reduces the conductivity of the monofilament (Figure 9b). To develop the filament detector, a trade-off between good rheological and mechanical properties and a good sensitivity is needed. The figure of merit (F), which quantifies the mechanical and detection property, decreases slightly before dropping sharply for the blends with more than 30 wt.% PBE ( Figure 10). As all the properties, this fall corresponds to the phase inversion of the morphology between nodular and fibrillar. The figure of merit highlights the decrease of the elongation at break and the sensitivity to water with the addition of PBE in the blend. Moreover, to develop the detector filament using the melt spinning, the melt flow index (MFI) has to be around 20 to 25 g/10 min. The two formulations that correspond to these criteria are PA6.63CNT80/PBE20 and the PA6.63CNT70/PBE30. Therefore, the optimum CPC filament to optimize is revealed thanks to the figure of merit which is the PA6.63CNT70/PBE30.

Conclusions
This study is focused on the development and the characterization of an immiscible thermoplastic/elastomer blend for the water detection. The monofilament is created by

Conclusions
This study is focused on the development and the characterization of an immiscible thermoplastic/elastomer blend for the water detection. The monofilament is created by two successive extrusions: by filling the PA6.6 with CNT first, and then by adding the PBE in different blend proportions.
Regarding the rheological properties, the CNT increase the blend viscosity whereas the fluidity increases with the proportion of PBE. The observations have correlated the mechanical and water detection properties with the blends' morphology. Two morphologies are revealed by the SEM and TEM images: the nodular and fibrillar morphology.
Below 30 wt.% of PBE in the blend, the PBE is in the form of nodules dispersed in the PA6.6 3CNT . The elongation at break increases with the proportion of PBE, while the electrical conductivity slightly decreases. The sensitivity of the absorption principle and the conductance of the short circuit follow the same trend as the dry conductivity of the monofilament. The sensitivity has a positive change, which means that the conductivity of the detector monofilament increases with the water contact. It creates new conductive paths between fillers and water.
Above 30 wt.% of PBE, the morphology changes for a fibrillar form of the PA6.6 3CNT in the PBE. This loss of phase percolation corresponds to the decrease of the mechanical, electrically conductive and detection properties. Regarding the mechanical property, the bad interface between PA6.6 3CNT and PBE increases with the PBE proportion, which causes a premature break of the monofilament. Regarding the absorption principle, the sensitivity becomes negative. The electrical conductivity of the monofilament decreases with the contact with water due to the modification of the interparticular distance. To find a trade-off between good rheological property and good mechanical and water detection properties, the figure of merit shows that the best candidate to optimize is the monofilament of PA6.6 3CNT 70/PBE30. The objective is to increase and refine the sensitivity of the detection filament by passing this formulation in melt spinning. Future work aims to develop this multifilament to reduce the standard deviation of its water sensitivity and to verify its morphology and its electrical property.